1. Field of the Invention
The present invention relates to a semiconductor device for controlling a switching power supply whereby the output voltage of the switching power supply is controlled by a switching operation.
2. Description of the Related Art
Conventionally, switching power supply units using semiconductor devices for controlling switching power supplies are widely used as the power supplies of home appliances such as home electrical products to improve power efficiency with low power consumption. In the semiconductor device, an output voltage is controlled (stabilized) by using the switching operation of a semiconductor (a switching element such as a transistor).
Particularly in recent years, in view of the prevention of global warming, attention has focused on a reduction of power consumption in a standby state of appliances such as home electrical products and thus switching power supply units achieving lower power consumption during standby are in great demand.
In order to meet the demand, a power supply system or the like is developed for properly using two switching power supply units according to an operation mode of an appliance. For example, a switching power supply unit for a main power supply is provided to supply power at rated load in a normal operation (normal mode) of the appliance, and a switching power supply unit only for a standby state is separately provided to supply power during standby in a standby operation (standby mode) of the appliance. When the appliance is in a standby state, power is supplied from the switching power supply only for a standby state. At rated load, power is supplied from the switching power supply for the main power supply.
However, this power supply system requires two switching power supply units (converters), increasing the cost of the overall circuit including the semiconductor devices for controlling the switching power supplies. Therefore, when lower cost is strongly demanded, a power supply system constituted of a single switching power supply unit (converter) has been used in many cases. In this case, switching power supply units of partial resonance have been frequently used in view of the efficiency of a power supply and noise.
In such a semiconductor device for controlling a switching power supply, a current applied to a switching element is reduced at a light load, e.g., during standby. However, it is always necessary to supply, via a transformer, the internal circuit current of the semiconductor device for controlling the switching power supply. Therefore, it is not possible to reduce a current flowing to the switching power supply to 0 as well as a current flowing to the switching element and thus a certain amount of current is applied even at no load. Hence, the switching operation of the switching element causes a loss even at no load and a lighter load causes a larger loss in the switching element. Consequently, the switching power supply decreases in power efficiency and thus it is not possible to meet the need for lower power consumption in a standby state of the power supply.
Further, the switching power supply of partial resonance has the following problem: since the switching power supply has a high oscillation frequency at light load, a switching loss increases and the efficiency of the power supply decreases in a standby mode.
(Prior Art 1)
As a solution for the low efficiency of a power supply in the standby mode (e.g., Japanese Patent Laid-Open No. 2002-315333), the following controlling technique is used: the state of a load on the secondary side of the power supply is detected by a microcomputer, a transition is made to a standby mode in response to the signal, and intermittent oscillation is performed by feedback control according to a commercial frequency. In this case, in order to improve the efficiency of a power supply in a stand by mode, feedback control is performed by the microcomputer as follows: when an output voltage increases to a predetermined value or higher at light load, the switching operation of a switching element is stopped, and then when the output voltage decreases to the predetermined value or lower, the switching operation of the switching element is restarted.
In this switching power supply, an oscillation frequency in the intermittence of the switching operation is constant regardless of a load. Thus, improvement in the efficiency of the power supply during standby is still insufficient.
(Prior Art 2)
For this problem, a switching power supply unit is devised. Referring to FIG. 16, the switching power supply unit will be schematically discussed below.
FIG. 16 is a circuit diagram showing a structural example of the conventional switching power supply unit. As shown in FIG. 16, in the switching power supply unit, a switching power supply applies a direct-current input voltage VIN to a switching element 1 via a primary winding 103a of a transformer 103, controls a direct-current output voltage Vo by the switching operation of the switching element 1, and supplies power to a load 109. The output voltage Vo is obtained by rectifying and smoothing an alternating current, which has been generated on a secondary winding 103b of the transformer 103, by a rectifier 104 and a capacitor 105. The switching power supply comprises: a transformer reset detection circuit 13 which detects the reset state of the transformer 103 according to an alternating voltage generated on a tertiary winding 103c of the transformer 103 and outputs a transformer reset detection signal indicating the reset state, the reset state being caused by the switching operation of the switching element 1; an I-V converter 29 for converting a change in control current, which is obtained through an output voltage detection circuit 106 and a phototransistor 110 according to a change in the direct-current voltage Vo generated on the secondary winding 103b of the transformer 103, into a voltage corresponding to the value of current; and a light load detection circuit 32 for outputting a control signal for controlling the intermittent operation of switching performed by the switching element 1 when a light load is detected according to a change in output voltage VEAO from the I-V converter 29 as a load state indicating power supplied to a load 109. These circuits and converter constitute a part of a control circuit for driving the control electrode (gate electrode) of the switching element 1.
When the output voltage VEAO from the I-V converter 29 is lower than a light load detection lower limit voltage VR1 for detecting a light load, the light load detection circuit 32 stops the switching operation of the switching element 1. When the output voltage VEAO from the I-V converter 29 is higher than a light load detection upper limit voltage VR2 for detecting a light load, the light load detection circuit 32 outputs a control signal for controlling an intermittent operation so as to restart the switching operation of the switching element 1. The control circuit drives the control electrode (gate electrode) of the switching element 1 in response to the transformer reset detection signal from the transformer reset detection circuit 13 and the control signal from the light load detection circuit 32 to control the intermittent operation at light load.
The following will describe the schematic operations of the switching power supply unit configured thus. The following will describe the power supply operation of a semiconductor device for controlling a switching power supply which performs, when a light load is detected, the intermittent operation of switching performed by the switching element.
In FIG. 16, when the internal circuit increases to a reference voltage, the control circuit is started. Thereafter, when the voltage of a terminal 46 is increased by a capacitor 118 connected between a terminal 46 and a terminal 47 to reach a start voltage, the switching element 1 such as a power MOSFET is turned on. When the drain current of the switching element 1 reaches an overcurrent detection level which is determined by the feedback current of a photocoupler current to a phototransistor 110 from an output voltage detection circuit 106 connected to the secondary winding 103b of the transformer 103, the switching element 1 is turned off. When the switching element 1 is turned off, the drain voltage of the switching element 1 causes a ringing operation due to resonance between the inductance of the transformer 103 and a capacitance between the drain and source of the switching element 1.
When the semiconductor device for controlling the switching power supply is started once, the subsequent on signal is detected by the tertiary winding (bias winding) 103c of the transformer 103. The voltage of the bias winding is clamped at + to − level in the control circuit. When the voltage of the bias voltage is equal to or lower than a set value in the control circuit, the on signal is outputted. A resistor 116 and a capacitor 117 are connected to a bias winding detecting terminal 49. Time constants determined by the values of the resistor 116 and the capacitor 117 are adjusted so as to obtain timing to turn on the switching element 1 at the bottom of the drain voltage of the switching element 1.
These operations are repeated to obtain a desired output voltage Vo. In order to improve the efficiency of the power supply at light load, intermittent oscillation control (intermittent switching operation) is performed such that a feedback current equal to or higher than a certain value stops the switching operation of the switching element 1 and a feedback current equal to or lower than a certain value restarts the switching operation of the switching element 1. Thus, it is possible to improve the efficiency of the power supply at light load and reduce power consumption.
The controlling method of the switching operation of the switching element 1 is suitable for a market requiring low noise, high efficiency, and high output. This is because the controlling method is RCC control of quasi-resonance, a switching loss can be reduced when the switching element is turned on, and low noise is achieved. Further, since an intermittent switching operation is performed by intermittent oscillation control at light load, it is possible to suppress an increase in switching frequency, which generally causes a problem in RCC, at light load. A switching loss at light load is reduced to a certain degree.
However, in the conventional switching power supply unit, a return signal for restarting the switching operation in the intermittent switching operation is outputted according to a feedback current value. It is not possible to recognize the level of the drain voltage of the switching element when switching is restarted by the return signal, resulting in hard switching at the restart of switching. Thus, a switching loss occurs when the switching element is turned on.
Further, a capacitor of a relatively large capacitance is connected as the capacitor 118 between the drain and source of the switching element. Thus, when the switching element is turned on, a loss of CV2/2 occurs. As a drain voltage increases, a larger loss is generated by the capacitor 118.
For these reasons, it is not possible to reduce the current loss of the switching element at light load or obtain sufficiently high power efficiency over a wide load area including a standby mode, thereby interfering with low cost and improvement in the efficiency of the switching power supply.